1. Field of the Invention
This invention relates to an optical receiver specifically designed for a fiber optic system. More particularly, it is especially suitable for a digital fiber optic system operating at high pulse rates, such as for example time multiplexed communications systems.
2. Background Information
A common photodetector used in fiber optic systems to convert light signals to electrical signals is the PIN diode because of its sensitivity, linearity, and relatively low cost. Two types of circuits are generally used with PIN diodes to produce the pulsed electrical output in digital fiber optic systems; the equalization type receiver and the transimpedance type receiver. In any such system it is desirable to have a wide dynamic range and wide bandwidth, but with low noise.
In the equalization type receiver, the current signal generated by the PIN diode is converted to a voltage signal by applying the output of the diode to a resistor. For a wide bandwidth the value of the resistor must be low, however, this increases the noise content of the voltage signal. To improve the signal to noise ratio, the equalization type receiver circuit uses a large resistor to convert the current signal to a voltage signal, however, this reduces the bandwidth. An equalizer stage later restores the bandwidth.
In the transimpedance type receiver circuit, a high gain amplifier with an impedance connected between the amplifier input and output terminals is used to convert the current signal generated by the PIN diode into a voltage signal. This arrangement allows a wide bandwidth for a large value of feedback resistance which reduces the noise content of the voltage signal, but has such a large dynamic range that the signals of lower magnitude are insufficient to operate a comparator provided in the output circuit to restore the shape of the received pulses. Accordingly, an automatic gain control is often inserted between the voltage to current converter and the comparator in the output stage. However, such an automatic gain control is complex and may be too slow for rapid changes in signal level.
In order to obtain wide bandwidth with a large feedback resistor in the transimpedance amplifier, the amplifier input capacitance must be as low as possible. It is known to utilize low capacitance devices connected in a cascode configuration to avoid the increase in input capacitance due to the Miller effect. However, to our knowledge such cascoded transistor stages have not been used in current to voltage converters for transimpedance type optical receivers. In any event, cascoded transistor devices to date have been, to our knowledge, of the same type: that is both FETs or both bipolar transistors, and have either both been PNP devices or both NPN (both N-channel or both P-channel devices for FETs).
In view of all of the above, there remains a need for an inexpensive fiber optic receiver which has the required sensitivity, bandwidth and low noise level for receiving data at high pulse rates such as the 1.544 mega bits per second pulse rate of T1 communications systems.
Accordingly it is the primary objection of the invention to provide a broad band, wide dynamic range, low noise optical receiver for pulsed light signals which is inexpensive.